There are broad industrial requirements for smoothing processes. In general industrial smoothing processes are in the following classes:
a) mechanical polishing--friction abrasion polishing and lapping using harder materials or abrasives such as in polishing of optical lens and metallic reflectors.
b) chemical bath etching--general industrial process to smooth parts after machining employing chemically active solvents.
c) plasma sputtering and etching--partially ionized gases sputter away high points or creation of volatile reaction products by ion bombardment can be used for extensive use in semiconductor manufacture for many purposes including smoothing.
In connection with the manufacture of semiconductor microelectronic devices, specialized smoothing processes have become important in support of several aspects of the manufacture of those devices. In particular, as semiconductors have become more densely populated, they have required multiple overlying layers of metal interconnections which need to be electrically isolated by multiple layers of dielectric materials. Because the processes employ photolithographic techniques to pattern the metal interconnects, it is essential that the surface be as flat as possible so that the depth of field of the optical imaging system results in very precise patterns, i.e., sharp line images across the entire wafer. This requires flatness on the order of 25.ANG. across the wafer. To meet these requirements, new smoothing processes for semiconductor processing has advanced the art of smoothing.
The microelectronic smoothing processes to advance photolithography are now called "planarization" because of the flatness requirement over the dimension of the diameter of the wafer. The most common current semiconductor planarization process employs a specialized plasma etching process. In this process, the low spots in the wafer surface are first filled with a thin spun on layer of a flowable and hardenable material, which material has an etch rate which is close to the etch rate of the interlayer dielectric material. Next, the thin layer is hardened and the wafer is plasma etched. Usually the etch is continued until the higher regions and the spin-on-layer is substantially entirely removed. Frequently, this process is repeated several times. This procedure is known as the "Double Etch-Back Process."
The Double Etch-Back Process provides good smoothing for processes having line widths greater than 1 micron but has not been entirely satisfactory because of the shrinkage of the volume of the flow-on-material when it is hardened. Accordingly, in those regions of the wafers in which the higher regions are more widely separated, a deep sag can occur which is difficult to planarize even with repeat of the process.
To overcome these problems, a chemical-mechanical so called "global" process having larger scale planarization is becoming common. The chemical-mechanical process is a derivative of the older lens polishing procedures except that a liquid chemical etchant for the dielectric is substituted for the abrasives of the earlier processes. In this process, the wafer surface is pressed against a rotating slightly rough surface with the noncompressible liquid etchant between them. This process combines a mild mechanical friction action along with the accelerated chemical action on the roughened high spots to achieve broad "global" planarization. This chemical-mechanical process is expensive and time consuming to implement because each wafer must be individually handled and processed. Additionally, the process is not as selective as desired because the chemical etchant reacts with almost equal rate on the lower regions as on the high.